A wide variety of processes use radial or horizontal flow reactors to effect the contact of particulate matter with a gaseous stream. These processes include hydrocarbon conversion adsorption and exhaust gas treatment. In most of these processes, contact of the particulate material with the fluid decreases the effectiveness of the particulate material in accomplishing its attendant function. In order to maintain the effectiveness of the process, a system has been developed whereby particulate material is semi-continuously withdrawn from the contacting zone and replaced by fresh particulate material so that the horizontal flow of fluidized material will constantly contact particulate material having a required degree of effectiveness. Typical examples and arrangements for such systems can be found in U.S. Pat. No. 3,647,680, U.S. Pat. No. 3,692,496 and U.S. Pat. No. 3,692,496 and U.S. Pat. No. 3,706,536.
A good example of the way in which moving bed apparatus has been used for the contacting of fluids and solids is found in the field of petroleum and petrochemical processes especially in the field the hydrocarbon conversion reactions. A well-known process that uses a radial flow bed for the contact of solid catalyst particles with a vapor phase reactant stream is found in the reforming of naphtha boiling hydrocarbons. This process uses one or more reaction zones where the catalyst particles enter the top of the reactor and flow downwardly under gravity flow and are transported out of the first reactor. In many cases, a second reactor is located either underneath or next to the first reactor. Catalyst particles again move through the second reactor under gravity flow. After passing through the second reactor, the catalyst particles can go through a further series of reaction zones and are collected and transported to a regeneration vessel for the restoration of the catalyst particles by the removal of coke and other hydrocarbon by-products that are produced in the reaction zone and accumulate on the catalyst. In the reforming of hydrocarbons using the moving bed system, the reactants typically flow serially through the reaction zones. The reforming reaction is typically endothermic so the reactant stream is heated before each reaction zone to supply the necessary heat for the reaction. The reactants flow through each reaction zone in a substantially horizontal direction through a bed of catalyst. In most cases the catalyst bed is arranged in an annular form so that the reactants flow radially through the catalyst bed. Many other hydrocarbon conversion processes can also be effected with a system for continuously moving catalyst particles under gravity flow through one or more reactors having a horizontal flow of reactants. One such process is the dehydrogenation of paraffins as shown in U.S. Pat. No. 3,978,150. The catalyst particles in each reaction zone are retained between an inlet screen and an outlet screen that together form a vertical bed and allow the passage of vapor through the bed.
Experience has shown that the horizontal flow of reactants through the bed of catalyst can interfere with the gravity flow removal of catalyst particles. This phenomenon is usually referred to as hang-up or pinning and it imposes a constraint on horizontal flow reactor designs. Catalyst pinning occurs when the frictional forces between catalyst pills that resist the downward movement of the catalyst pills are greater than the gravitational forces acting to pull the catalyst pills downward. The frictional forces occur when the horizontal flow vapor passes through the catalyst bed. When pinning occurs, it traps catalyst particles against the outlet screen of the reactor bed and prevents the downward movement of the pinned catalyst particles. In a simple straight reactor bed, or an annular bed with an inward radial flow of vapors, pinning progresses from the face of the outlet screen and as the vapor flow through the reactor bed increases, it proceeds out to the outer surface of the bed at which point the bed is described as being 100% pinned. Once pinning has progressed to the outermost portion of the catalyst bed, a second phenomenon called void blowing begins. Void blowing describes the movement of the catalyst bed away from its outer boundary by the forces from the horizontal flow of vapor and the creation of a void between the inlet screen and the outer catalyst boundary. The existence of this void can allow catalyst particles to blow around or churn and create catalyst fines. Void blowing can also occur in an annular catalyst bed when vapor flows radially outward thorough the bed. With radially outward flow, void blowing occurs when the frictional forces between the catalyst pills are greater than the gravitational forces, or in other words, at about the same time as pinning would occur with a radially inward flow. Therefore, high vapor flow can cause void blowing in any type of radial or horizontal flow bed.
The production of fines can pose a number of problems in a continuous moving bed design. The presence of catalyst fines increases the pressure drop across the catalyst bed thereby further contributing to the pinning and void blowing problems. Catalyst fines can also accumulate in the narrow openings of the screen surfaces used to retain the catalyst particles thereby plugging these surfaces and requiring a shut-down of the equipment to remove catalyst fines. Catalyst fines are usually more abrasive than the larger catalyst particles and thereby contribute to greater erosion of the process equipment. Finally, the catalyst in many of these hydrocarbon conversion processes is a valuable commodity and the generation, and removal of catalyst fines imposes a direct catalyst cost on the operation of the system. Further discussion of catalyst fines and the problems imposed thereby can be found in U.S. Pat. No. 3,825,116 which also describes an apparatus and method for fines removal.
Where possible, horizontal or radial flow reactors are designed and operated to avoid process conditions that will lead to pinning and void blowing. This is true in the case of moving bed and non-moving bed designs. Apparatus and methods of operation for avoiding or overcoming pinning and void blowing problems are shown in U.S. Pat. No. 4,135,886, 4,141,690 and 4,250,018.
In some processes, particularly the dehydrogenation of paraffins, radial flow bed designs are used with very high velocities that produce a catalyst bed that is more than 100% pinned and in most cases will exceed 200% pinned. Thus, these beds are subjected to void blowing as well as pinning. However, operational experience with these types of beds have been satisfactory despite the existence of void blowing due to the very high gas velocity which prevents the generation of fines and the discontinuous operation of these processes. The very high gas velocities used in these processes is believed to prevent the erosion of catalyst after the formation of voids by imposing sufficient drag forces on individual catalyst particles to keep the individual particles from blowing around or churning within the void. Hence, erosion is not a problem when voids are formed if the gas velocity is sufficient to hold individual particles against the free surface of the void. In addition to the high gas velocity preventing erosion, the discontinuous nature of operation as typically practiced in these processes also avoids the accumulation of any fines that might be generated since the process is operated until the catalyst is completely replaced so that the majority of any fines that might be generated are also removed at the time of catalyst replacement. Therefore, in a discontinuous process having a very high gas velocity across the particle bed, the generation of fines may not pose significant problems. However, it would be highly desirable to have an apparatus that would prevent void blowing and thereby permit the use of a moving particulate bed in processes that use a high gas velocity.
Accordingly, it is an object of this invention to provide an apparatus for a gravity flow moving particle bed that prevents the occurrence of void blowing.
It is a further object of this invention to reduce the generation of fine particulate matter in the operation of a fluid solid contacting bed that uses gravity flow to remove and replace particles.
It is a more specific object of this invention to provide an apparatus for the dehydrogenation of paraffinic hydrocarbons by the continuous contact of paraffin vapors with particulate catalyst in an apparatus having a continuous withdrawal and replacement of catalyst particles under gravity flow.